1. Field of the Invention
This invention relates generally to paper transport systems and more particularly to a transport system relying on demarcations sensed on the paper to accurately position the paper relative to a print head. The term, "paper" as used throughout this specification denotes any suitable recording medium for printing text or graphics, regardless of substance or size.
2. Description of the Prior Art
Line printers and plotters are widely used to produce computer-based hard copy output as a result of their relative simplicity and low cost. In the prior art, these print mechanisms have been characterized by the controlled movement of the to-be-printed medium in a machine direction past a stationary or translating print head or pen which is caused to print on the paper in the transverse direction. Depending on the nature and construction of the inking system, including the intended printing sequence, the paper may be controlled to move continuously, incrementally or bidirectionally in the machine direction with respect to the print head or pen.
Conventional paper transports for printer mechanisms may be generally divided into friction and tractor drives. Friction drives pinch the paper between a driving drum or roller and a pinch roller(s), and are used primarily for advancing single cut sheets of paper through the printer. In contrast, tractor drives are designed to advance continuous lengths of paper via a sprocket which engages a series of holes located at even increments along one or both edge-margins of the paper, i.e. beyond (outside) the print area. In the latter case the length of the paper is usually perforated to permit removal of the outer sprocket holes and for separation of the individual sheets of paper following printing.
In the prior art, tractor drives have been used primarily for printing text in conjunction with a unidirectional paper transport system which typically exhibits relatively low registration accuracy. Cumulative coupling error is attributable to built-in clearances between the sprocket teeth and holes, error due to mismatch between sprocket hole centers, and shifting due to hole enlargements or deformation resulting from sprocket engagement. Typically the maximum positioning accuracy of a sprocket drive transport is in the order of several thousandths of an inch.
Equivalent paper coupling errors are avoided in friction drives due to the direct surface coupling of the capstan with the paper. Prior art friction paper transports have exhibited paper position accuracy as small as 1/1000 of an inch. Coupling errors in friction drives arise from manufacturing tolerances on the friction wheel radius and the trueness (roundness) and tolerances of reducing elements between the friction wheel and the controlling motor, as well as rubber compression and varying media coupling onto the roughened surface.
The coupling error and limitations of the prior art are overcome by the subject invention through the use of a novel position feedback mechanism. Micrometer positioning accuracy is offered in embodiments for both tractor and friction drive transports. The invention may be used to virtually eliminate mechanical registration error contributing to fringe banding, whereby higher speed precision electronic printing is made possible for high-resolution, linear reproduction (copy) response for both black-and-white and color dot matrix hard copy output.
Various solutions to achieving precision registration of a printer paper about a print head have been pursued by the graphics industry, the most relevant of which is represented by the Versatec Model--four color printer, manufactured by Versatec Corporation of Santa Clara, CA. The system employs pre-printed margin demarcations having an interdependent spatial relationship, wherein appendent sensor means is used to detect each of said demarcations in order to compensate for positioning error due to paper shrinkage. A positioning signal responding to the measured distance between said demarcations is used to activate a repeated multi-pass paper advance after the paper is initially conditioned through a first pass. A major disadvantage of the Versatec printer that is overcome by this invention is that the Versatec system advances the paper based on the absolute distance between pre-printed demarcations as opposed to relative positioning of the print head as taught herein. Accordingly, the Versatec system, while superb for a continuous motion advance in conjunction with a suitable capstan drive, is not nearly as effective with paper requiring single-line indexing or swath printing. For example, in the case of a tractor feed drive, as changes in the spacing between the holes in the paper due to elongation or tears, affecting paper registration, may result in perceived "banding" due to cumulative positioning error that cannot be adequately reduced or compensated by averaging techniques to allow for changes in the distance between the pre-printed demarcations used to measure paper shrinkage. In contrast to the limitations of the Versatec printer, the relative positioning taught herein compensates for both coupling error and paper shrinkage. In addition to inherent limitations of coupling error, the Versatec system prints colors serially, i.e., one line at a time whereas this system provides for multiple line printing with a single swath or pass. Accordingly, colors or lines of print are registered with each other via the print head. Thus, "fringe banding" a visual phenomenon inherent in the teachings of the prior art is overcome by this invention.
Such fringe banding is especially a problem at borders and edges or in multi-color printing. Such errors can come from a multitude of sources including changes in the effective radius of the drive wheels; errors caused by the drive reduction mechanism; windup tolerances in the motor shaft and drive belts; and deadband errors which may occur in the encoder, or due to friction.
FIG. 11 illustrates the potential sources of positioning error in a drive system wherein a capstan 1 and pinch roller 2 advance the paper on which printing is carried out. It should be noted that the drive radius varies with the thickness of the media, since it is measured from the centerline of the capstan 1 to the centerline 3 of the print media 4.
The following example data represent the degree and source of cumulative coupling error that is responsible for the perceived "banding" which limits the resolution of conventional printers. These data are based on a typical 0.50" dia direct 0.382" capstan drive advancing 0.125" per step, and lead to the cumulative error calculated below. These data assume a nominal paper thickness within the tolerances indicated. A major source of coupling error (not included in the table) is the variation in the selected paper thickness which represents addition to effective coupling error not compensated or addressed in prior art printers.
Within the thickness range of typical commercial papers, e.g. 2 to 8 mils, the potential contribution to the effective coupling error is a factor of two times the cumulative error from all other mechanical sources. The required positioning accuracy (in the order of several micrometers) to eliminate the nuisance of fringe banding is not possible or advanced in the teachings of the prior art for precision printers or plotters.
______________________________________ COUPLING ERROR Contribution Tolerance to total step Source of Error (+/- inch) error (+/-um) ______________________________________ Capstan: Radius 0.0001 1.27 Out of round 0.00005 0.64 Riding Radius (Paper): Engagement 0.001 12.70 Paper Thickness 0.0005 3.18 Reduction Coupling: Coupling N/A 0 (gear noise) Coupling N/A 0 ratio tolerance Deadband: Encoder resolution 4000 pulses/rev 5.00 Frictional -- 2.00 Backlash None 0 Windup -- 1.00 ______________________________________ Representative Total Error: +/- 26 um (+1-.001")
Systems incorporating coarse and fine positioning according to existing demarcations on film strips (i.e. the sprocket holes) have been disclosed by Field, et al.--see Patentschrift DE 27 22 378 and Brevet d'Invention No. 77 13050.
However, while offering means for precision frame registration for high resolution cameras and projectors, the perceived cumulative error in extending the line width of the recording medium from a narrow film strip to an 80-column paper and assumed coupling error makes the transfer of technology difficult and not obvious. For example, misregistration of successive line print creates fringe banding or density variation due to inking which is not perceived or addressed in the registration of picture frames for projection or camera copy. Also in the case of film, the problem of frame by frame registration is directed at the successive overlay of a moving image and frame to create the illusion of steadiness, whereas in the case of printing the problem of registration is directed at repeating the registration within a constant frame image. Accordingly, precision paper registration transport employing the features of relevant prior art in apparatus developed for the motion picture industry has not been adopted or heretofore advanced for application to a precision paper drive system for printers or plotters.